The present invention relates to a gas turbine having moving blades cooled with a coolant and, more particularly, to a coolant collection type gas turbine provided with flow paths for cooling moving blades inside the gas turbine rotor and made so as to collect the coolant after cooling the moving blades.
Moving blades of a gas turbine, usually, are cooled with air supplied through an interior of a rotor in order to protect them from a high temperature combustion gas flowing in a combustion gas path (hereunder referred to as a gas path). Usually, a part of compressed air for combustion is used as a source of the air and discharged into the gas path after cooling.
In gas turbines the higher the combustion gas temperature, the higher the efficiency. However, since a thermal load increases by making the combustion gas temperature higher, a flow rate of a cooling air is inherently increased. Discharging the cooling air into the gas path not only decreases the temperature of combustion gas, but disturbs a flow in the gas path and decreases the output performance of the turbine. Further, a coolant flowing in a circulating flow path in the rotor has revolution energy proportional to 2 powers of the radius, however, discharging the coolant from the moving blades arranged on the outer periphery of the rotor causes a lot of pumping power loss, and the loss increases proportionally to a flow rate of the coolant. Therefore, an effective efficiency improvement can not be expected by only making the temperature of combustion gas higher.
In order to improve further the performance, it is necessary to collect the air supplied for cooling the moving blades to solve the above-mentioned problems.
Therefore, for example, in a gas turbine disclosed in JP A 54-13809, a method of forming a course of supplying and collecting a coolant with piping inside the rotor is proposed, and in a gas turbine disclosed in JP A 3-275946 a method of forming a course of supply and collection of a coolant by perforating the inside of the rotor is proposed.
Further, in the gas turbine disclosed in JP A 7-189739, an axial collection flow path is formed at a stacking connecting portion of a turbine rotor, and it is made so that the air after cooling blades is collected in the combustion chamber through the collection flow path.
For constructing a coolant collection type gas turbine, it is necessary to form inside a turbine rotor a supply flow path for supplying a coolant for moving blades and a collection flow path for collecting the coolant after cooling. However, since cooling the moving blades increases the temperature of the coolant used therefor, thermal stresses occur in the rotor components having both the supply flow path and the collection flow path because of difference in coolant temperature.
In a gas turbine of 1500.degree. C. class combustion gas temperature, since the temperature of a coolant rises to about 200-250.degree. C. in the case of air cooling and to about 150-200.degree. C. in the case of vapor cooling, excessively large thermal stresses beyond an allowable value may occur depending on construction of a flow path. Therefore, in order to realize a coolant collecting gas turbine of high efficiency by making the combustion gas temperature higher, it is desirable to form a coolant supply flow path and a coolant collection flow path inside the rotor so as to reduce thermal stresses.
Further, in a coolant collecting gas turbine of an air cooling type in which moving blades are cooled by using a part of compressed air for combustion and the air after cooling is collected in a combustor, it is necessary to raise a collection pressure to at least a discharge pressure of the compressor. Therefore, the coolant is pressurized by a boost compressor before being supplied. However, because of high temperature, increasing a flow rate of the coolant inherently increases compression power of the boost compressor, so that it influences greatly the efficiency of the whole gas turbine system. Therefore, in order to attain an expected high efficiency, it is necessary to devise a flow path construction which is able to extremely reduce a pressure loss of the coolant flowing inside the rotor. Those points are not considered in any of the prior art.
In the closed cooling gas turbine disclosed in JP A 7-189739 or JP A 9-13902, both a supply flow path and a collection flow path for cooling blades are disposed inside a rotor, so that thermal stress occurs in construction components of the rotor because of the above-mentioned temperature elevation of coolant.
Thermal stress is influenced by the structure of a rotor and a flow path, and the thermal stress also can be reduced by measures for thermal shielding. However, since the rotor is a rotary body rotating at a high speed, the structure thereof is limited with respect to the strength. Although it is possible to form relatively easily a flow path at a stacking connecting portion, including the thermal shielding measures, the flow path construction has many problems such that branching and joining are necessary for forming communication flow paths between the flow path and blades at an outer periphery because many blades are connected, and stress is apt to increase upon perforation of the flow path because the disk has a thin thickness at the outer periphery side. Further large thermal stress occurs due to temperature difference between the side faces of the disc when counter flow is effected between coolant supply and coolant collection on the both side faces of the disc.
Further, a supply temperature of a coolant, of about 250.degree. C. is effective in view of operation of the gas turbine system, and in this case, a collection temperature is 400-500.degree. C. Thereby, the temperature of some portions goes beyond an allowable temperature of a turbine rotor material usually used, so that it is necessary to use a material of high heat resistance and high cost.